Radar is used in many applications to detect target objects such as airplanes, military targets, and vehicles. A relatively recent application for radar is in radar systems for automobiles. Automotive radar systems are known for use in helping drivers to park their cars, to follow traffic at a safe distance, and to detect driving obstacles. In such applications, when the radar system detects an obstacle or the slowing down of traffic in front of the vehicle, it may issue a warning to the driver, such as a beep or warning light on the dashboard, and/or actually control the vehicle in some way, such as by applying the brakes, in order to avoid an accident.
Typically, a radar system detects the range (i.e., distance) to a target object by determining the roundtrip delay period between the transmission of a radar signal and the receipt of the signal returning back to the radar after it bounces off of the target object. This round-trip delay, divided in half and then multiplied by the speed of the radiation, c, gives the distance between the radar system and the target object (assuming the transmitting antenna and the receiving antenna are the same antenna or very close to each other). The location of the target object typically is determined in one of three general ways. In one technique, the radar beam is narrow such that the bearing to the target object is given by the direction in which the beam is pointing, in both azimuth and elevation, when it sends out a signal that results in a return signal reflected off of a target object. The combination of the bearing, the elevation, and the range gives the location of the target object. In the second technique, a plurality of radar transmitters and/or receivers can be arranged in an array and the range information collected from the multiple transmitters and/or receivers can be correlated with each other to determine the location of the target object via trilateration. Particularly, the knowledge of the range of a target object from a known location (e.g., a particular receiver) defines a sphere on which the target object must lie, that sphere being the sphere that is centered at the radar receiver and that has a radius equal to the measured range. By determining the range sphere for a single target object from a plurality of different receivers at different locations, one can calculate the locus of points where all of those spheres intersect. In many practical applications as few as three receivers is sufficient to provide enough data to uniquely identify a single point at which the target object lies. In a third technique for detecting the azimuth and elevation of a target object known as monopulse radar technique, the relative amplitudes of the reflected signal from two or more radar antennas, with common coverage in azimuth, elevation and range, are used to determine azimuth and elevation angles.
Although many radars locate target objects in range, azimuth (or bearing), and elevation, typical automotive radars ignore elevation. However, nuisance objects that cause undesired radar detections (such as extremely low target objects that a car could easily traverse) may require some discrimination in elevation or height.
One example where elevation discrimination may prove useful is in automotive radar systems used for obstacle avoidance, where it would be desirable to determine the height of a target object in front of the vehicle and toward which the vehicle is moving. Particularly, target objects below a certain height are probably of no danger to the vehicle or its occupants if the vehicle were to drive over that target object, whereas obstacles over a certain height probably would be dangerous to drive over or into. For instance, small objects such as aluminum cans and other small debris usually are perfectly safe for the vehicle to run over. In fact, any contoured object of significant mass or electromagnetic reflectivity (such as metal objects) could return a reflection to the radar system even when they are virtually at the ground level. Such obstacles include manhole covers and expansion joints in roadways.
Accordingly, it would be desirable for radar systems to discriminate between target objects based on the height. For example, it would be desirable for a vehicle radar system to distinguish between low target objects and high target objects so as to not issue a warning or apply the brakes unnecessarily frequently with respect to objects that are not truly obstacles or otherwise dangerous to the vehicle or its occupants.